X-values in circuit design simulations represent unknown signal values, such as “don't care” values or “wildcard” values. These different interpretations of the X-values may cause different behaviors in physical circuits and simulations. Additionally, standard hardware description language (HDL) simulation semantics conceal X-value related bugs (X-bugs) in simulation as these X-values may not be propagated in certain scenarios. These X-bugs are generally uncovered during gate level simulation, which comes late in a circuit designing cycle and causes added cost in fixing the X-bugs. However, with the use of X-propagation in simulation, X-bugs can be detected at a register transfer level (RTL), which comes before gate level simulation. Although X-bugs are commonly detected at an earlier state of the circuit designing cycle, identifying the failure point of the X-bugs can be a cumbersome and time consuming process.
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